1. Field of the Invention
The present invention relates to a method of forming a deposited film, and more particularly to a method of forming a deposited film for use in the manufacture of a device containing amorphous silicon (hereinafter called "a-Si") as a component thereof.
2. Related Art
Hydrogenated amorphous silicon (a-Si:H) semiconductors have been widely used in electronic devices, such as electrophotographic photosensitive members, image pickup tubes, solid-state image sensing devices, TFTs for display, solar cells, and so forth.
Hitherto, a variety of methods for depositing an amorphous semiconductor of a type containing elements in Group IV of the Periodic Table, such as a-Si, a-SiGe, a-SiC, a-SiN, or a-SiO, have been suggested as exemplified by a vacuum evaporation method (see, for example, Journal of Non-Crystalline Solids, Vols. 8 to 10, p. 739, 1972), a sputtering method, or any of various CVD methods, such as a plasma CVD method and a photo CVD method.
Among the methods, a plasma CVD method using a semiconductor precursor gas, such as SiH.sub.4, to form a film on a substrate by RF glow discharge decomposition employing a frequency of 13.56 MHz has been widely used because excellent quality a-Si can be deposited.
The plasma CVD method developed by R. Chittick et al. (Journal of Electrochemical Society, Vol. 166, p. 77, 1969), and W. E. Spear et al. (see Solid State Communications, Vol. 17, p. 1193, 1975) succeeded in providing pn-control of the electric conduction occurring due to impurities in the amorphous semiconductor, attracting great attention. Thus, a multiplicity of applications have been attempted, such as solar cells (see Japanese Patent Laid-Open No. 52-16990 and so forth) and electrophotographic photosensitive members (see Japanese Patent Laid-Open No. 54-86341 and so forth). The foregoing methods have been widely developed, resulting in a variety of modifications as exemplified by a method wherein a semiconductor precursor gas is diluted with hydrogen or Ar, a method in which a frequency higher than RF is used, and a method in which a magnetic field is applied in order to create electron cyclotron resonance.
Superior characteristics (photoconductivity and controllability of the electric conductivity with impurities and so forth) to that obtainable from pure a-Si can be obtained because a-Si prepared by the plasma CVD method contains several to tens of % hydrogen (see Applied Physics Letters, Vol. 30, No. 11, p. 561, 1977) and 10 to 20% hydrogen is normally contained in the deposited film. A variety of methods have been developed in which hydrogen is simultaneously supplied during formation of the film by other methods.
For example, supply of hydrogen during a vacuum evaporation method enables excellent a-Si to be obtained (Journal of Applied Physics, Vol. 49, No. 12, p. 6192, 1978).
An attempt has been made to perform a sputtering method using Ar plasma and a semiconductor target made of Si or SiGe in such a manner that hydrogen gas is mixed with Ar gas serving as discharge gas, and a target is caused to react with an element to be deposited with the target material sputtered by high-frequency plasma of 13.56 MHz, to deposit an amorphous film on an oppositely facing substrate, i.e. reactive sputtering (see, for example, Solid State Communications, Vol. 20, p. 969, 1976). It has been found that the foregoing method deposits an excellent quality a-Si:H film with a very low number of dangling bonds.
The sputtering method has an advantage that semiconductor precursor gas, the cost of which cannot be reduced and which is dangerous and cannot be preserved for a long time, is not required. Therefore, its peripheral apparatuses, such as a large cost and large scale toxin-removing apparatus, gas-leakage alarm system and cylinder cabinet, can be simplified considerably, thereby reducing the cost.
Improvement in the foregoing method such that formation of an a-Si film by a so-called bias sputtering method comprising the step of applying a bias to a substrate has been investigated as disclosed in AIP conference Proceedings, Vol. 73, p. 47 (1981) and Solar Energy Materials, Vol. 8, p. 187 (1982) and the like.
As disclosed in Japanese Patent Laid-Open No. 4-65120 or Japanese Journal of Applied Physics, Vol. 30, No. 5B, p. 1881 (1991) or Applied Physics Letters, Vol. 59, No. 9, p. 1096 (1991), some methods have been suggested in which formation of a very thin deposited film by the plasma CVD method followed by irradiation of the deposited film by plasma without formation of the deposited film are alternately performed.
However, the conventional methods have some problems. The high frequency plasma CVD method using SiH.sub.4 gas which has been usually used to form a-Si deposits satisfactory a-Si:H.
However, the conventional plasma CVD method using the reactive gas, such as SiH.sub.4, necessitates a large cost discharged gas toxin-removal apparatus, gas leakage prevention facility and a gas leakage alarm system. Moreover, high-purity semiconductor gas, the cost of which cannot be reduced and which cannot be preserved for a long time, is required. In addition, the efficiency of the gas usage is very low, for example, about 10% and the film formation rate is unsatisfactory. Therefore, cost reduction cannot be realized satisfactorily.
If excellent quality is intended in the formed film, a low deposition rate is generally required. Thus, it has been difficult to realize simultaneously high deposition rate, excellent film quality, and a large area.
Moreover, a large quantity of the a-Si film adheres to surfaces other than the substrate, such as the inner wall of the film forming chamber and the substrate holder when the a-Si film is formed, causing the a-Si film to be gradually separated, thereby deteriorating the characteristics of the device. In order to prevent this, the a-Si film allowed to adhere to the inner surface of the chamber must be removed periodically. The removing operation must be performed frequently, causing the availability of the film forming apparatus to be lowered.
On the other hand, the reactive sputtering method in which hydrogen is mixed with discharge gas composed of Ar or the like and which uses a high-frequency plasma of about 13.56 MHz does not use large cost and dangerous semiconductor gas. Therefore, the toxin-removing apparatus and the gas leakage alarm system and so forth can be omitted from the structure or can be decreased. Furthermore, the necessity of using the large cost gas can be eliminated, as a result of which great cost reduction can be provided. Moreover, adhesion of films to the inner surface of the film forming chamber can be relatively reduced, and, therefore, maintenance of the chamber can easily be performed.
Although the foregoing advantages can be obtained, the a-Si film formed by the conventional reactive sputtering method involves the following disadvantages. When Ar is mixed with the film, the film is damaged by Ar.sup.+ ion bombardment, and the film is formed in a columnar structure so that a uniform and precise film cannot easily be formed. Therefore, the a-Si film is inferior to that formed by the plasma CVD method in that the spin density is relatively high, electricity conductivity (.sigma..sub.p) under light irradiation is low, carrier migration characteristics are low, efficiency of doping impurities is low, the deposited films have a columnar non-uniform structure in the direction in which the film grows, and several percent of Ar gas is incorporated in the film. Thus, use of the a-Si film of the foregoing type in a device encounters these problems.
The difference between the characteristics of the a-Si film formed by the reactive sputtering method and those of the a-Si film formed by the plasma CVD method have been reported in a multiplicity of disclosures, for example, Japanese Journal of Applied Physics, Vol. 19, Supplement 19-1, p. 521 (1980).
Although the cause of the foregoing problems has not been clarified yet, it may be considered that they are due to the following causes. First, the deposited film is damaged by ions emitted from the plasma which bombards the surface of the substrate, on which the film is growing and which is placed to face the target.
The sputtering process generally uses a discharge plasma of inert gas of, for example Ar, having a relatively large atomic weight in order to improve the sputtering yield of the target. Among DC discharge, AC discharge, and high-frequency discharge for generating the plasma, it is preferable that high-frequency discharge be employed to form a high-resistance material, such as a-Si. Although ions, such as Ar.sup.+ ions, in the high-frequency plasma are accelerated due to the spontaneous bias or a target bias applied externally, colliding with the target to cause atoms in the target to be ejected forcibly, they travel as well to the surface of the growing film. As a result, the deposited film is damaged.
This is due to the potential difference between the plasma and the surface of the growing film, that is, the so-called sheath voltage. The potential difference accelerates the positive ions, such as Ar.sup.+ ions, in the direction of the substrate, causing the deposited film on the substrate to be damaged.
A method of preventing the damage by controlling the substrate bias has been known. As disclosed in, for example, Journal of Vacuum Science and Technology, Vol. 14, p. 92 (1977) or AIP Conference Proceedings, Vol. 73, p. 47 (1981) or Solar Energy Materials, Vol. 8 p. 187 (1982), attempts have been made to apply a bias voltage to the substrate to control the energy of Ar.sup.+ ions incident from the plasma to the surface of the growing film.
For example, the characteristics of an a-Si:H film deposited on a substrate by using, for example, a polycrystal Si target and a RF discharge (13.56 MHz) using Ar gas containing hydrogen, are affected by the substrate bias. For example, hydrogen in a film deposited by applying a positive substrate bias forms SiH bonds characterized by an infrared absorption peak near 2000 cm.sup.-1. Although films of a type formed by a GD method usually exhibit excellent characteristics, such as a high .sigma..sub.p, the sputtering method cannot always cause the film to have excellent characteristics. There are sometimes formed non-uniform films having unsatisfactory characteristics including a columnar structure.
It has been reported that, if the substrate bias is negative or floating so as to apply a negative induced bias, the hydrogen bonds in the film are brought into a state where bonds, such as SiH.sub.2 and (SiH.sub.2).sub.n, characterized by an infrared absorption peak near 2100 cm.sup.-1, are increased and ion damage cannot be prevented, so characteristics may deteriorate. On the other hand, the columnar structure cannot easily be formed and therefore the uniformity of the film cannot always deteriorate.
Another problem is that Ar atoms can easily be captured by the film because Ar.sup.+ ions are injected into the film. The injection of the Ar.sup.+ ions into the deposited film raised the concentration of Ar atoms received by the film, causing an adverse effect to arise. At least a portion of the effect of the substrate bias is due to the action of the Ar.sup.+ ions incident upon the surface of the growing film during the deposition of the film. Another cause may be electrons made incident upon the substrate during deposition. The mechanism has not been clarified yet, and control of the quality of the film has not been obtained.
Another reason is that the ion energy distribution in the plasma is very wide. The wide distribution is caused by acceleration of Ar.sup.+ ions and so forth in the plasma due to the high-frequency AC field. Since Ar.sup.+ ions and Ne.sup.+ ions only narrowly follow the changes in the electric field at a high frequency of about 13.56 MHz, acceleration due to it undesirably takes place. As a result, ions having considerably high initial speeds are formed. Therefore, even if the sheath voltage is, as the DC component, 0 V, high-energy components of ions reach the surface of the deposited film. As a result, it is difficult to completely prevent damage of the deposited film caused by ions by controlling the substrate bias.
There also is a problem due to the migration characteristics of Si atoms on the surface during the film deposition. When a-Si is deposited by the reactive sputtering method using inert gas and hydrogen gas, a major portion of Si sputtered from the target which travels to the surface of the substrate is Si atoms, ions such as Si.sup.+ ions, radicals such as SiH, and their ions. The foregoing substances generally have high chemical reactivity as compared with SiH.sub.3 which is a precursor of a reaction in the RF plasma CVD method using SiH.sub.4. Therefore, if the foregoing active substances exhibiting excellent reactivity reach the surface of the substrate having relatively low temperature, the active substances immediately react substantially as they are. Film deposition proceeds although the structure relaxation process, such as surface transference during the film deposition, has not been completed. As a result, the deposited film is formed into a structure having a multiplicity of structural defects.
The foregoing problem is not critical with metal, such as Al, Cu, Ti or the like, because the bonding orienting force is weak and changes in the structure cannot easily appear in the characteristics. However, it is a very important factor when a film of semiconductor material, such as a-Si, the covalent characteristics of which are strong, the directionality of the bonds of which is clear and the structure sensitivity of which affects the characteristics, is formed. In particular, a-Si is, among amorphous materials, a semiconductor, the electricity conductivity of which can be pn-controlled by means of impurities, and whose characteristics have structural sensitivity in proportion to the quality of the film. Therefore, influence of the deterioration in the surface migration characteristics of Si atoms and H atoms appears as well as the influence of the ion damage, causing .sigma..sub.p to be deteriorated, the doping efficiency in pn-control of the electricity conductivity by doping impurities to be lowered, and the uniformity of the film to be worsened. That is, it is important for a semiconductor material, such as a-Si or a-Si alloy, to enhance the surface migration characteristics of deposited atoms while preventing ion damage of the deposited film.
Therefore, in a case where an excellent a-Si film is deposited by the sputtering method, it is effective to enhance the surface migration characteristics of the reactive precursor on the surface on which the film is growing by any method which promotes structure relaxation in the deposition process.
In the conventional plasma CVD method and the reactive sputtering method, the foregoing requirement has been achieved to a certain degree because the surface, on which the film is growing, is deactivated by hydrogen. Although it can obtain deactivation of the surface by means of hydrogen, the considerably high reactivity of the reactive substances, which reach the surface, causes etching to take place. As a result, the surface can be roughened. Moreover, hydrogen is excessively incorporated in the film, resulting in a structure, such as SiH.sub.2 bonds, which worsens the quality of the film, and the precision of the film formation deteriorates. Therefore, simple supply of hydrogen in a large quantity cannot meet the requirement. Although control of the quantity of hydrogen in the reactive sputtering method can be accomplished by employing control of the partial pressure of hydrogen gas or the like, it has been difficult to obtain an excellent a-Si film.
There is a possibility that the surface migration characteristic of Si atoms can be enhanced by raising the temperature of the substrate. However, the foregoing method is caught in the dilemma that excessive increase of the substrate temperature (about 350.degree. C. or higher) causes H atoms adsorbed on the surface to enhance their surface migration characteristics to start separation therefrom and consequently the surface characteristics of Si atoms deteriorate. If the temperature of the substrate is too high, the concentration of hydrogen in the film is lowered, causing the amount of dangling bonds to be increased. Therefore, the characteristics of the film deteriorate. As a result, the foregoing method has a limitation.
An attempt has been made to form a precise structure by bombarding the surface of the growing film by ions in the plasma. If the ion irradiation is performed with a negative substrate bias during the film deposition, the columnar structure which is frequently formed in the sputtering method is not formed and the quality of the film can be improved (Solar Energy Materials, Vol. 8, p. 187, 1982). However, the quality of the film sometimes deteriorates by the applied bias, and it cannot easily be controlled. One reason is that the film is damaged by ions.
Metal deposition methods have been disclosed in, for example, Japanese Patent Publication No. 3-65013, each of which comprises the step of lowering the frequency of a high-frequency bias applied to a substrate to widen the ion energy distribution to prevent formation of projections and pits in the surface of the deposited film. According to the disclosure, plasma, the frequency of which is 5 kHz to 1 MHz, is employed to apply ions, the energy of which is distributed widely, to the substrate to enhance the surface migration characteristics of atoms in the formed film and flatten the deposited film. Although flattening of the surface of the deposited metal film is discussed in the foregoing disclosure, a similar effect, limited to flattening, can, of course, be expected qualitatively in the case of Si. However, the thus-deposited flat a-Si film has no semiconductor characteristics, such as pn-control of the electrical conductivity and the photoconductivity, which are important for applications. This is because the characteristics of a semiconductor material used to form a film, such as Si, have intense structural sensitivity and can easily be affected by the ion damage.
In the conventional method, such as the GD method, the quantity of hydrogen contained in an excellent a-Si film is not less than tens of %. A film of the foregoing type has disadvantageous characteristics, such as deterioration in the film quality due to light, which is caused by weak Si-Si bonds generated due to containing hydrogen. The conventional method inevitably increases the amount of dangling bonds if the concentration of hydrogen is forcibly lowered by, for example, raising the temperature of the substrate. In this case the quality of the film deteriorates.
For preventing the foregoing problem, a method has been disclosed in Applied Physics Letters, Vol. 59, No. 9, p. 1096 (1991) in which formation of a deposited film and irradiation of the film with a hydrogen plasma are alternately repeated. The foregoing method exhibits an advantage in that the dependency of the concentration of hydrogen in the film on the temperature of the substrate can be reduced. The quality of the obtained film does not deteriorate even if the concentration of hydrogen is lowered.
However, the necessity of switching the discharge gas raises a limitation on the speed of switching from the film deposition to irradiation of hydrogen plasma. Therefore, a time during which the gas flow rate and the plasma conditions cannot be controlled is generated between two film deposition steps interposing a hydrogen irradiation step. As a result, the stability of the film deposition sometimes deteriorates. If a certain level of discharge electrical power is supplied at the usual discharge frequency of about 13.56 MHz, the quality of the film growing on the surface sometimes deteriorates due to ion damage.